1. Field of the Invention
This invention relates to an image processing method for improving the quality of an image to be displayed on a display device and to a liquid-crystal display device using the same.
2. Description of the Related Art
FIG. 33 shows an example in a structure of a liquid crystal display device of vertically aligned type. FIG. 33A typically shows a sectional structure of a liquid crystal panel 101. The liquid-crystal panel 101 is constructed by a TFT substrate (array substrate) 102 formed with Thin-film transistors (TFTs), etc., an opposite substrate 103 formed with a common electrode and a CF (color filter), and a liquid crystal 104 sealed between those by attaching through a peripheral seal material 105. Between the TFT substrate 102 and the opposite substrate 103, a cell gap is maintained at a predetermined spacing by a spacer 106. Polarizer plates 107 are respectively provided, for example, in a cross Nichol arrangement on the opposite surfaces of the TFT substrate 102 and the opposite substrate 103 to the facing surfaces. Meanwhile, a mounting terminal 108 is formed on the TFT substrate 102, to mount thereon an IC (not shown) for driving the liquid crystal.
FIG. 33B shows a structure of one pixel 113 in a state the liquid-crystal display device of vertically aligned type is viewed in a direction of the normal to a display surface thereof (hereinafter, referred to as “in a frontward direction”). A pixel electrode pattern for driving the liquid crystal is formed on at least one of the substrates, e.g., TFT substrate 102. A plurality of drain bus lines 111 and gate bus lines 112 are formed crossing through an insulation film over the TFT substrate 102, at the interconnection of which are formed pixel-driving TFTs 110 connected with respective pixel electrode 109. Furthermore, each pixel 113 has a storage capacitor electrode 116 for storing charge. Also, the storage capacitor electrode 116 has a lower layer formed with a storage capacitor bus line 117 through an insulation film.
A slit 114 is formed by removal of an electrode material on the pixel electrode 109 while a linear protrusion 115 is formed on the opposite substrate 103 side. The slit 114 and the protrusion 115 serve as an alignment regulating structure for regulating the direction in which the liquid-crystal molecules (not shown) of the liquid crystal 104 are to tilt under the application of voltage. Within the pixel, the domain is partitioned to allow the liquid-crystal molecules to tilt in four directions. By allowing the liquid molecules to tilt in four directions, the deformation in viewing angle is averaged as compared to that of the liquid-crystal display device having a tilt only in one direction. This greatly improves the characteristic of viewing angle. This technology is called alignment partitioning art.
FIG. 34 typically shows a sectional structure of a liquid-crystal display device of vertically aligned type using an alignment partitioning technique. In FIG. 34A, the alignment regulating structural protrusion 115 is formed on both of a pixel electrode 109 film-formed over the TFT substrate 102 and an opposite electrode 118 film-formed over the opposite substrate 103. An alignment film 119 is formed over the TFT substrate 102 and the opposite substrate 103 including over the protrusion 115. Incidentally, although not shown, the protrusion 115 in some cases is provided on one substrate only. FIG. 34A shows a state that voltage is not applied to the liquid crystal 104. FIG. 34B shows a state that voltage is applied to the liquid crystal 104 wherein liquid-crystal molecules 120 are aligned in two directions. Meanwhile, FIG. 34C shows a state that the slit 114 is provided only on the TFT substrate 102 wherein voltage is applied to liquid crystal 104. In this case also, the liquid-crystal molecules 120 are aligned in two directions. Incidentally, the slit 114 in some cases is provided only on the opposite substrate 103 or on both of the TFT substrate 102 and the opposite substrate 103.
Meanwhile, different from the LCD shown in FIGS. 33 and 34, there exists a liquid-crystal display device for a mode that liquid-crystal molecules 120 are nearly parallel with the TFT substrate 102 in the initial state under no application voltage to the liquid crystal 104 but the liquid-crystal molecules 120 rise when voltage is applied. Such liquid-crystal display devices include the TN (Twisted Nematic) type, as an example. In the TN type, a rubbing process is previously performed over the alignment film formed on the TFT substrate 102 and opposite substrate 103, to determine an alignment direction of the liquid-crystal molecules 120. This accordingly does not require slits 114 and protrusions 115. However, for alignment partitioning, there is a need to separate the tilt direction of the liquid-crystal molecules 120 into a certain number. It is a practice to realize alignment partitioning by locally changing the pre-tilt. Besides the TN type, there are various liquid-crystal display modes including IPS (In-Plane Switching) having liquid crystal molecules 120 that do not tilt relative to the TFT substrate 102, ferroelectric liquid-crystal and so on. However, in other liquid-crystal modes other than the IPS and ferroelectric liquid-crystal, there is a common problem of poor viewing-angle characteristic.
FIG. 35 is a figure explaining a problem involved in the liquid-crystal display device on the conventional driving scheme. FIG. 35A shows a characteristic (T-V characteristic) of an application voltage to liquid-crystal layer versus transmissivity on a liquid-crystal display device of vertically aligned type. In the graph, the curve A shown by the solid line having plotting with solid circle marks represents a T-V characteristic in the frontward direction while the curve B shown by the solid line having plotting with asterisk marks represents a T-V characteristic in a direction of azimuth 90 degrees and polar angle 60 degrees relative to the display screen (hereinafter, referred to as “oblique direction”). Here, azimuth is assumable an angle as measured counterclockwise from nearly a center of the display screen with reference to the horizontal direction. Meanwhile, polar angle is an angle defined with a vertical line taken at the center of the display screen.
In the part shown by a virtual circle C in FIG. 35A, there is caused a distortion in luminance change. For example, with a comparatively low luminance at an application voltage of approximately 2.5 V, transmissivity is higher in the oblique direction than in the frontward direction. However, with a comparatively high luminance at an application voltage of approximately 4.5 V, transmissivity is lower in the oblique direction than in the frontward direction. As a result, there is a decrease in the luminance difference within the range of effective drive voltage when viewing in the oblique direction. This phenomenon is to appear the most conspicuous as color changes. Namely, when viewing the display screen obliquely relative to the frontward direction, there is a change of color into white. FIG. 35B represents a tone-level histogram of red (R), green (G) and blue (B) of a video image taken from the front and in the oblique by a digital camera under the same condition. The abscissa represents a tone level (e.g., luminance increases as closer to 0, with 256 levels of 0-255) while ordinate represents an existence percentage (%). It can be seen that, in the frontward direction, the R, G, B distributions are distant from one another whereas, in the oblique direction, the distributions are closer to one another. Due to this, the color in nature is lost.
The methods for improving this phenomenon are disclosed in Patent documents 1 to 7. FIG. 36 shows a basic pixel structure shown in Patent Document 1. FIG. 36A represents a typical view of a pixel structure taken in a normal-line direction to the display screen, FIG. 36B represents an equivalent circuit of a pixel 121 and FIG. 36C represents a sectional structure of the pixel 121. As shown in FIG. 33B, usually one pixel electrode 109 is connected to one TFT 110. However, as shown in FIG. 36A, one pixel is split into four sub-pixels 121a, 121b, 121c and 121d. The sub-pixels 121a, 121b, 121c and 121d are electrically in a relationship of capacitance coupling. When voltage is applied to the pixel 121 through the TFT 110, charge is distributed in accordance with the capacitance ratio of the sub-pixels 121a, 121b, 121c and 121d thus applying different voltages to the sub-pixels 121a, 121b, 121c and 121d. Due to this, the distortion on the T-V characteristic shown in FIG. 35A is dispersed by the sub-pixels 121a, 121b, 121c and 121d, thereby moderating the white on the screen. Incidentally, the principle of dispersing the distortion in T-V characteristic will be referred to later. Hereinafter, the method of splitting the pixel 121 into the sub-pixels 121a, 121b, 121c and 121d is referred to as an HT (halftone grayscale) technique based on capacitance coupling. The HT technique based on capacitance coupling is applied to the display mode of the TN type liquid-crystal display.
[Patent Document 1]
JP-A-3-122621
[Patent Document 2]
JP-A-4-348324
[Patent Document 3]
JP-A-5-66412
[Patent Document 4]
JP-A-5-107556
[Patent Document 5]
JP-A-6-332009
[Patent Document 6]
JP-A-6-519211
[Patent Document 7]
JP-A-2-249025
In the HT technique based on capacitance coupling, the pixel structure is extremely complicated. First, one pixel must be split into a plurality of pixels. In case the sub-pixel is poor in pattern going into a contact, a point defect results. Meanwhile, for capacitance coupling, there is a necessity to arrange three-dimensionally the sub-pixels 121a, 121b, 121c and 121d between the opposite electrode 118 and the controlling capacitor electrode 122 formed on the TFT substrate, as shown in FIG. 36C. In the case of an occurrence of short circuit between layers or the like, the entire pixel goes into a point defect. Meanwhile, in the case when capacitance distribution is changed by pattern breakage or so forth, luminance is changed in the entire pixel. In this case, point defect is encountered. Furthermore, splitting as sub-pixels greatly reduces the opening ratio. The HT technique based on capacitance coupling unavoidably suffers the reduction in opening ratio. In order to moderate the opening-ratio reduction to a possible minimum extent, there is a need to make transparent the two layer electrodes forming the capacitance. In this case, because the process increases in film deposition, there is encountered a great effect upon the process, e.g., mounting up of manufacturing cost, process capability lowering, etc.
Meanwhile, the HT technique based on capacitance coupling involves the problem that drive voltage is required to be high. This is attributable to a voltage loss caused in capacitance coupling, i.e., higher drive voltage is required as the number of split sub-pixels increases. Higher drive voltage requires increasing consumption power. Furthermore, high breakdown strength of a drive IC is required which raises cost. Also, because the HT technique based on capacitance coupling is provided with a potential difference by the sub-pixels, the T-V characteristic combined is non-continuous. Display characteristic is inferior to that in the ideal state where the T-V characteristic is continuous in change.
As in the above, although the HT technique based on capacitance coupling has an effect to improve display characteristic, it is not adopted for the liquid-crystal display devices presently available in the market. Meanwhile, the TN liquid-crystal display device, as viewed obliquely, problematically has increased intensity of black thus lowering contrast. The HT technique based on capacitance coupling is an art to correctly represent a neutral tonal intensity. However, under reduced contrast, it is impossible to exhibit the color representation effect at a neutral-tone intensity level.